Light sensitive bimetallic control switch



Sept. 27, 1966 P. J. CUR-HN 3,275,832

' LIGHT SENSITIVE BIMETALLIC CONTROL SWITCH Filed Oct. '7, 1965 2 Sheets-Sheet 1 il@ ff@ if d Sept. 27, 1966 P. J. cURTlN 3,275,832

LIGHT SENSITIVE BIMETALLIC CONTROL swITcH Filed Oct. 7, 1965 2 Sheets-Sheet 2 'I INVENToR. Paz/ J 607mb? United States Patent O 3,275,832 LIGHT SENSITIVE BIMETALLIC CONTROL SWITCH Paul J. Curtin, South Milwaukee, Wis., assignor to McGraw-Edison Company, Milwaukee, Wis., a corporation of Delaware i FiledA Oct. 7, 1963, Ser. No. 314,269 Claims. (Cl. Z50- 206) This invention relates to light sensitive switches and more particularly to light sensitive control switches for outdoor luminaires of the type employing a light sensitive resi-stance means for controlling current flow to a heater resistor disposed in close proximity to a switch actuating bimetallic element.

Prior art photo-control devices of the resistance heater type generally employ a fine insulate-d resistance wire which is wound around the birnetallic switch actuating element. Such windings are costly to manufacture, are structurally weak and are susceptible to surge voltages.

Another shortcoming of some prior art Iphoto-control switches employing cadmi-um suliide photo-control elements is that such elements can be damaged by relatively low surge voltages. This is due to the fact that in the absence of light such elements have iniinite resi-stance so that substantially the entire surge voltage on the unit will be impressed across the photo-sensitive element.

It is an object of the invention to provide a photoc-ontrol device having a .switch actuating bimetallic element which is provided with a bight portion in close proximity to a carbon resi-stance heater. Another object of the i11- vention is to provide a discharge tube for protecting the photoasensitive eleme-nt in such photo-control devices from voltage surges.

A further object of the invention is to provide a photocontrol unit with `a new and improved resilient overeenter switch. A more specitic object of the invention is to provide an overcenter switch for a photo-control device having a fixed contact and a pivotally mounted switch arm which is coupled by resilient means to the free end of a bimetall-ic element so that flexure of the element in one direction will cause the switch to snap open `and exure in the opposite direction will snap the switch to a closed position.

These and other objects and advantages of the instant invention will become more apparent with the detailed description thereof taken with the accompanying drawings in which:

FIG. 1 is a circuit diagram schematically illustrating the photo-control device according to the instant invention;

FIG. 2 is a top plan view of the photo-control switch according to the instant invention;

FIG. 3 is a view taken along lines 3 3 of FIG. 2;

FIG. 4 is a view taken along lines 4 4 of FIG. 3;

FIG. 5 illustrates the operation of the bimetallic element and the overcenter switch ot the device shown in FIG. 2; and

FIG. 6 is a View taken along lines 6-6 of FIG. 5.

Referring now to the drawings in greater detail, PIG. 1 shows the circuitry of the photo-control' device 10 operative to connect the lamp 12 being controlled to an energy source 13. The photo-control includes a cadmium sulfide photo-cell 1 4 and a heater resistor element 15 which are connected in series with each other and to the energy source 13 by conductors 17 and 18. A bimetallic element 20 is placed in thermal .association with the heater resistor 15 and is operative when sufficiently heated to open a normally closed snap acting switch 21. When the element is relaxed, the switch 21 is closed to connect the lamp 12 to the energy source 13 through conductors 17, 18, 22 and 23, which are connected as 3,275,832 Patented Sept. 27, 1966 "ice shown to the input terminals 25, 26, and 3'9 of the control 10.

The leads 27 and 218 of a neon discharge lamp 24 are respect-ively connected to the terminals 56 and 47 of the phototcell 14 to provide a shunt discharge path so that if the control device 10 is subjected to surge voltages the tube 24 will break down and by-pass the cell 14. In this regard, the heater resistor 15 yacts as a current limiting resistor to insure that the current ilowing through the neon lamp 24 will not be destructively high.. In addition, because the res-istor 15 can withstand substantially higher surge voltages than wire wound resistors, use of the former eliminates the necessity of additional surge protection across the line terminals as required in some prior .art devices.

The control 10 is shown in FIGS. 2, 3, and 4 to include a generally planar base 30 of any suitable insulating material, such as Bakelite, and having a generally L- shaped conductive bracket 31 aiixed thereto in any suitable manner, such as by screws 33. The conductive bracket 31 also carries a terminal member 26 for connection to the line conductor 18. A pair of insulating blocks 36 are atiixed to the bracket 31 by screws 37. Rigidly clamped between the block-s 36 is a second terminal member 39 which is connected to the lamp wire 22 and which also carries the stationary contact 40 of the snap switch 21. As seen in FIG. 2, the screws 37 pass through an enlarged clearance hole 42 inthe terminal member 39 without engagement therewith so that there is no electrical current pat-h between the terminal member 39 and the bracket 31. The screws 37 do, however, electrically engage the bracket 31 and also aiiix one end of the bimetallic element 20 to the insulating blocks 36 opposite the bracket member 31.

The bimetallic ele-ment 20 is shown in FIG. 2 to have a generally U-shaped yconfiguration with a generally semicircular bight 44 formed adjacent its fixed end. The carbon resistor 15 is shown to have a cylindrical conliguration and to be disposed parallel to and within the bight 44 yand in close proximity of the bimetal-lic element 20. The gap between the resistor 15 and the bight 44 may be llled with high temperature silicone grease to improve the transfer of heat therebetween.

The photocell 14 is shown in FIGS. 3 and 4 to include a cadmium suli'ide resistor element 50 disposed in a iiat circular housing 51 which is suitably aiiixed to the base plate 30 and has an aperture 52 covered by a transparent crystal 53 for exposing the element 50 to the ambient light. An insulating cloth 55 is disposed between housing 51 and the base plate 30 to provide electrical insula'tion.

One terminal 45 of resistor 1.5 is connected to one ofthe screws 37 so that it is also electrically connected to the supply wire 18 through terminal 26. The other terminal 46 of the resistor 15 is connected to one terminal 47 of the photocell 14. The other terminal 56 of the photocell 14 is connected to the terminal member 25 which is also connected to the supply conductor 17. Thus the current path through the photo-control is from the supply wire 18 to the terminal member 26, the screw 37, the heater resisto-r 15, the photocell 14, the terminal member 25 and the supply conductor 17.

Referring now specifically to FIGS. 5 and 6, an L-shaped conductive bracket member 60 is shown affixed ot the outer surface of the terminal member 26 and extending first toward the free end 61 of the bimetallic element 20 and then at a right angle toward the other side =of the U-shaped bimetallic element 20. A pair of notches 64 are formed in the opposite sides of the end 62 of member 61 for receiving the bifurcated end 65 of a contact arm 67 which carries a contact member 72 at` its other end and which forms the movable portion of the switch 21. In addition, the free end 62 -of member 60 has a central aperture 68 to permit the passage of an overcenter spring 70 which is connected at one end to the free end of the bimetallic element 61 and at its other end to the contact arm 67 at a point adjacent the contact member 72. The spring' 70 urges the bifurcated end of the contact arm 67 into high pressure engagement with the end portion 62 of the bracket 60 to provide good electrical engagement therebetween. When the bimetallic element is in its unfiexed position as shown in FIG. 2, the contact 72 carried by the c-ontact arm 67 enga-ges the stationary contact 40 so that a current -path is completed between input terminals 26 and 39 through the bracket 60, contact arm 67, and the contacts 72 and 40.

The operation Iof the snap switch 21 is illustrated in FIG. 5 which shows by full lines the bimetallic element 20 in its unexed condition and with snap switch 21 closed. This is the condition when the lamp 12 is being energized. Under conditions of darkness, the light impinging on the photocell 14 is insufficient to maintain its resistance at a low level so that the amount of current fiowing to the heater resistor 15 is insufficient to cause flexing of the bimetallic element. Should the light level increase sufiiciently to cause the resistance of the photo- `cell to diminish, a greater current will flow through the heater resistor 15 causing it to begin flexing the bimetallic element 20. As the element flexes from its position shown by full lines in FIG. 5 to its position shown by phantom lines, the end of the overcenter spring 70 connected to its free end 61 will begin moving past a line containing the axis -of the contact arm 67. As soon as the center line of the spring passes the axis of arm 67, the arm will be snapped counterclockwise as viewed in FIG. 5 to move the contact 72 away from the contact 40 thereby Vinterrupting the lamp circuit.

Should the level of illumination decrease, the resistance of photocell 14 will increase to diminish the current flowing through the resistor 15 whereupon it will begin cooling and the bimetallic element will return to its unflexed position. As soon as its free end 61 moves to the point where the center line of the spring 70 passes the axis of the arm 67, said arm will snap clockwise from its phantom position in FIG. 5 to its full position to close the contacts 72 and 40 and complete the energizing circuit to the lamp 12.

It will be appreciated that because the heating of resistor 15 and the fiexing of bimetallic element 20 do not occur instantaneously the photocell Will be immune to transient light changes such as lightning and the like.

The U-shaped configuration of the bimetallic element '20 provides ambient temperature compensation in that any ambient temperature changes which tend to flex the fixed end downwardly as viewed in FIG. 5 will simultaneously cause the free end 61 to flex upwardly so that the net movement of the overcenter spring 70 will be negligi-ble.

In order to calibrate the photocell 10 so that a greater or lesser deflection of the bimetallic element 2l) will cause the contacts 21 to be actuated, an adjusting screw 74 is carried by a relatively stiff bracket member 75 which is attached to the terminal 31 in a substantial parallel relation to the bracket 6i). The end of the screw 74 bears against the side of the bracket 60 so that turning of the screw will cause the bracket 60 to flex in one direction or the other thereby moving the end of the contact arm 67 relative to the free end of the bimetallic element 61. As a result, a greater or lesser amount of deflection of bimetallic element 20 will be required to move the spring 70 overcenter relative to the axis of the arm 67. A second screw 76 disposed adjacent to the screw 74 acts as a stop for the movable Contact 72 so as to adjustably set the length of the contact gap.

While only a single embodiment has been shown and described,'it is not intended that the application be limited thereby but only by the scope of the appended claims.

I claim:

1. In a photo-control device having a pair of input terminals, a photo-sensitive resistor and an elongate heater resistor connected in series, said photo-sensitive resistor being characterized by an impedance that decreases with increasing levels of illumination, an elongate bimetallic element fixed at one end and .having a generally U-shaped portion, said bimetallic element also having a bight portion adjacent one end of said U-shaped portion and dis-- posed in close proximity to said heater resistor, a snap acting switch having a fixed contact and a contact arm mounted for movement into and out of engagement with said fixed contact, resilient means coupling the free end of said bimetallic element Aand said contact arm whereby liexure of said bimetallic element in one direction will cause said resilient means to snap said contact arm away from said fixed Contact and return movement of said bimetallic element toward its original position being opera'- tive to snap said contact arm into a closed position relative to said fixed contact.

2. In a photo-control device having a pair of input terminals, a photo-sensitive resistor and a heater resistor connected in series, said photo-sensitive resistor being characterized by an impedance that decreases with increasing levels of illumination, said heater resistor being of the carbon type and having a substantially cylindrical configuration, an elongate bimetallic element fixed at one end and having a generally U-shaped portion, said bimetallic element also having a bight portion adjacent one end of said U-shaped portion and disposed in close proximity to said heater resistor, a snap acting switch having a fixed contact and a contact arm mounted for movement into and out of engagement with said fixed contact, said switch means being operative when in a closed position to complete a circuit between said input terminals, spring means coupling the free end of said bimetallic element and said contact arm whereby flexure of said bimetallic element in one direction will cause said spring means to snap said contact arm away from said fixed contact and return of said bimetallic element toward its original position being operative to snap said contact arm into a closed position relative to said fixed contact, and discharge means connected in parallel with said photo-sensitive resistor.

3. In a photo-control device having a pair of input terminals, a photo-sensitive resistor and an elongate heater resistor connected in series, said photo-sensitive resistor being characterized by an impedance that decreases with increasing levels of illumination, a bimetallic f element having a fixed end and a free end, said bimetallic element also having a bight portion disposed in close proximity to said heater resistor, a snap acting switch including a fixed contact and an elongate contact arm having one end pivotally mounted adjacent the free end of said bimetallic element, said switch means being operative when in a closed position to complete a circuit between said input terminals, spring means extending ybetween the free end of said bimetallic element and said contact arm at a point remote from the pivotal mount-ing thereof, whereby flexure of said-biinetallic element in one direction will cause said spring means to move overcenter in a first direction to snap said contact arm away from said fixed contact and return of said bimetallic element toward Iits original position being operative to move said spring means overcenter in the opposite direction to snap said contact arm into a closed position relative to said fixed contact.

4 In a photo-control device having a pair of input terminals, a photo-sensitive resistor and a heater resistor connected in series, said photo-sensitive resistor being .characterized by an impedance that decreases with increasing levels of illumination, said heater resistor having a substantially cylindrical configuration, an elongate U- shaped bimetallic element having a fixed end and a free end, said bimetallic element also having a bight portion intermediate its ends and disposed in close proximity to said heater resistor, a snap acting switch including a fixed contact and an elongate contact arm pivotally mounted adjacent the free end of said bimetallic element and extending generally away therefrom, said switch means being operative when in a closed position to complete a circuit between said input terminals, spring means extending between the free end of said bimetallic element and said contact arm at a point remote from the pivotal mounting thereof, whereby iiexure of said bimetallic element in one direction will cause said spring means to move overcenter in a rst direction to snap said Contact arm away from said xed contact and return of said bimetallic element toward its original position `being operative to move said spring means overcenter in the opposite direction to snap said contact arm into a closed position relative to said fixed contact.

5. In a photo-control device having a pair of input terminals, a photo-sensitive resistor and a heater resistor connected in series, said photo-sensitive resistor being characterized by an impedance that decreases with increasing levels of illumination, said heater resistor being of the carbon type and having a substantially cylindrical configuration, a generally U-shaped bimetallic element having a fixed end and a free end, said bimetallic element also having a vbight portion intermediate its ends and disposed in close proximity to said heater resistor, a snap acting switch including a iixed contact and an elongate contact arm having one end pivotally mounted adjacent the free end of said bimetallic element and extending longitudinally away therefrom, said switch means being operative when in a closed position to complete a circuit between said input terminals, spring means extending between the free end of said bimetallic element and said contact arm at a point remote from the pivoted end thereof, whereby exure of said bimetallic element in one direction will `cause said spring means to move overcenter in a iirst direction to snap said contact arm away from said xed contact and return of said -bimetallic element to its original position being operative to move said spring means overcenter in the opposite direction to snap said contact arm into a closed position relative to said fixed contact, and neon tube means connected in parallel with said photo-sensitive resistor.

References Cited by the Examiner UNITED STATES PATENTS 2,906,846 9/ 1959 Witherspoon 200-122 X 3,048,833 8/1962 Bernheim Z50-239 X 3,233,068 2/1966 Allen 200--122 RALPH G. NILSON, Primary Examiner. W. STOLWEIN, Assistant Examiner. 

1. IN A PHOTO-CONTROL DEVICE HAVING A PAIR OF INPUT TERMINALS, A PHOTO-SENSITIVE RESISTOR AND AN ELONGATE HEATER RESISTOR CONNECTED IN SERIES, SAID PHOTO-SENSITIVE RESISTOR BEING CHARACTERIZED BY AN IMPEDANCE THAT DECREASES WITH INCREASING LEVELS OF ILLUMINATION, AN ELONGATE BIMETALLIC ELEMENT FIXED AT ONE END AND HAVING A GENERALLY U-SHAPED PORTION, SAID BIMETALLIC ELEMENT ALSO HAVING A BIGHT PORTION ADJACENT ONE OF SAID U-SHAPED PORTION AND DISPOSED IN CLOSE PROXIMITY TO SAID HEATER RESISTOR, A SNAP ACTING SWITCH HAVING A FIXED CONTACT AND A CONTACT ARM MOUNTED FOR MOVEMENT INTO AND OUT OF ENGAGEMENT WITH SAID FIXED CONTACT, RESILIENT MEANS COUPLING THE FREE END OF SAID BIMETALLIC ELEMENT AND SAID CONTACT ARM WHEREBY FLEXURE OF SAID BIMETALLIC ELEMENT IN ONE DIRECTION WILL CAUSE SAID RESILIENT MEANS TO SNAP SAID CONTACT ARM AWAY FROM SAID FIXED CONTACT AND RETURN MOVEMENT OF SAID BIMETALLIC ELEMENT TOWARD ITS ORIGINAL POSITION BEING OPERATIVE TO SNAP SAID CONTACT ARM INTO A CLOSED POSITION RELATIVE TO SAID FIXED CONTACT. 